Many commonly used medicines are photosensitizers. We are developing a quantitative methodoloy for testing the potential of drugs for inducing photochemical damage to serum proteins and thence to distant parts of the body. The photochemical properties of antidiabetic drugs of the sulfonylurea type are being studied. Absorption spectra of acetohexamide, hydroxyhexamide, chlorpropamide and tolbutamide show n -pi* structure in the lowest singlet state and hence, the lowest triplet state of these molecules may be photochemically active (as a hydrogen atom abstractor). The near absence of fluorescence from these drugs suggest efficient intersystem crossing and thus a predominance of triplet state. Experiments conducted with crystallized human serum albumin shown that a fragment of about 40,000 daltons is produced when the system containing 5 x 10 to the minus 4th power 4M drug and 0.7 mg/ml albumin is irradiated at 240 nm. Irradiation of albumin with the xanthine dye Rose Bengal showed a different type of damage process: the main albumin band is diminished and high-molecular weight polymers are produced, but the 40,000 dalton fragment is apparently either not produced at all, or else appears in the polymeric bands. Control experiments with albumin alone indicate production of the 40,000 dalton fragment. The possibility that albumin can serve as a carrier of drug-induced photodamage is presently being examined. Preliminary evidence using low density lipoprotein and sarcoplasmic reticulum vesicles as acceptor systems for transported photochemical damage are encouraging, although the effects are weak. The contribution of free-radical intermediates is being evaluated.